Smoke and heat detector unit

ABSTRACT

A unit for detecting the presence of smoke in the air and for detecting a high heat condition. The unit comprises a casing containing a light source, a light detecting photocell which receives light deflected from smoke particles in air flowing through the casing and a second compensating photocell which compensates for the effects of variable conditions such as light source voltage and temperature on the detection photocell. A control circuit in the casing includes a noise producing horn and is capable of actuating the horn to provide three readily distinguishable sounds indicating the presence of smoke above a threshold level, a high heat condition, and burnout of the light source, respectively.

[451 Apr. 29, 1975 1 1 SMOKE AND HEAT DETECTOR UNIT 176] Inventor:Gordon A. Roberts, 1509 Kearney Rd.. Ann Arbor. Mich. 48103 22 Filed:May 17. 1973 21 Appl. No.: 361,128

Related U.S. Application Data [63] Continuation of Scr. No. 72,627.Sept. 16. 1970.

abandoned.

[521 U.S. Cl. 250/565; 250/575; 250/210: 340/237 S. 356/207 [51] Int.Cl. G01n 21/26 [58] Field of Search 250/218, 210. 206, 573, 250/574.575. 564, 565', 356/206. 208. 207; 340/237 S [56] References CitedUNITED STATES PATENTS 2,537,028 1/1951 Cahusac et a], 340/237 S3.233.781 2/1966 Gruhbs 250/210 X $255,441 6/1966 Goodwin ct a1. 250/218X 3.319.069 5/1967 Vassil l 1. 250/218 3,421,157 1/1969 Atkins 340/2513.555.532 1/1971 White et a1. 250/218 X 3.624.629 11/1971 Donaldson340/251 3.657.737 4/1972 Hamm et a1. 340/237 S Primary E.raminer-WalterStolwein Attorney, Agent, or FirmOlsen and Stephenson {57] ABSTRACT Aunit for detecting the presence of smoke in the air and for detecting ahigh heat condition. The unit comprises a casing containing a lightsource, a light detecting photocell which receives light deflected fromsmoke particles in air flowing through the casing and a secondcompensating photocell which compensates for the effects of variableconditions such as light source voltage and temperature on the detectionphotocell. A control circuit in the casing includes a noise producinghorn and is capable of actuating the horn to provide three readilydistinguishable sounds indicating the presence of smoke above athreshold level. a high heat condition, and burnout of the light source.respectively.

13 Claims, 3 Drawing Figures INVENTOR SHEET 2 BF 2 YENIEDmz 9 ars BYflak ATTORNEYS SMOKE AND HEAT DETECTOR UNIT This is a continuation. ofapplication Ser. No. 72.627 filed Sept. 16. I970. now abandoned.

This invention relates generally to units for detecting and signalingthe existence ofsmoke or heat in room air above a predetermined level ofthe type disclosed in my earlier U.S. patent No. 3.383.670. The unit ofthis invention is an improvement on the unit shown in the aforementionedpatent. The unit of this invention includes the same general arrangementof a light source. a lens containing light transmission conduit. a lightinteraction region. a light receptacle. a photosensitive cell forreceiving light deflected by smoke in the light interaction region and aphotocell actuated horn disclosed in the aforementioned patent. Inaddition. the unit of this invention includes a second photosensitivecell adjustably mounted in the light receptacle and connected in analarm circuit to the first cell for adjusting the sensitivity of thecircuit. The cells receive comparable light intensity at the selectedsmoke threshhold point. namely. the smoke concentration point which itis desired to sense. and by virtue of this arrangement the cells havesimilar resistive temperature coefficients and time constants. Thiscooperative arrangement of the two cells compensates for variableconditions such as line voltage affecting the intensity of the lightsource and heat conditions affecting the resistivity of the cells whichwould otherwise adversely affect the sensitivity of the unit. Inaddition, the unit of this invention includes a control circuit for thesignaling noise source which provides for the emission of three readilydistinguishable sounds from the noise source to indicate the threeconditions which it is desired to sense in the unit. The noise source. aconventional horn. emits intermittent bursts of noise when the lightsource burns out to indicate to the user of the unit that the lightsource lamp must be replaced. A second continuous sound at substantialsound level is emitted from the born when the unit senses a high heatcondition. A third continuous sound which is readily distinguishablebecause of its raspy character is emitted from the horn when the unitsenses a threshhold smoke condition. Thus, the smoke and heat detectorunit of this invention is capable of continuous operation over aprolonged service life to continuously monitor heat and smoke conditionswithin desired sensitivity limits for fire detection purposes.

Further objects. features and advantages of this invention will becomeapparent from a consideration of the following description. the appendedclaims, and the accompanying drawing in which:

FIG. I is a front view of the unit of this invention with the front faceof the enclosing casing broken away and other parts shown in section forthe purpose of clarity;

FIG. 2 is an enlarged fragmentary sectional view of a portion of theunit of this invention as seen from substantially the line 2-2 in FIG.I; and

FIG. 3 is a diagrammatic illustration of the alarm circuit which forms apart of the detector unit of this invention.

With reference to the drawing. the detector unit of this invention.indicated generally at I0, is illustrated in FIG. I as consisting of acasing 12 having a front wall 14. a back wall 16. a bottom wall I8 and atop wall 20. An inlet opening 22 for room air is formed in the bottomwall I8 and an outlet opening 24 for this air is formed in the top wall20. Partitions in the casing 12 define a smoke detection chamber ofcompartment 26 disposed between the openings 22 and 24, a lighttransmission conduit 28 containing a lens 30. a lower light trap 32 andan upper light trap 34. The casing partitions also cooperate to form alight receiving compartment 36 and a compartment 38 in which a signalhorn 40 is located. For a purpose to appear presently. orifice plates 41and 42, provided with orifices 43 and 44, respectively. are mounted inthe light transmission con duit 28 to one side of the smoke detectionchamber 26. A light source 46 is positioned in the casing I2 at one endof the light transmission conduit 28. When the light source 46 isenergized. it projects a light beam 48 onto the lens 30 and the lens 30in turn projects this beam of light toward the orifice plate 42 so thatan image of the light source 46 is formed at the orifice 44 and adiverging beam of light is projected through the orifice 43 and acrossthe detection chamber 26. This diverging light beam forms a lightinteraction region 49 in the chamber 26 between the orifice 43 and aninlet open ing 50 for the light receiving compartment 36 and between theupper and lower boundaries 52 of the light beam.

A first photosensitive cell PCI is mounted in the easing I2 at one endofthe chamber 26 so that some of the light impinging on particles in theinteraction region 49 will be reflected onto the cell PCI. A secondphotosensitive cell PC2 is disposed in the light chamber 36 and ismounted on a pivot 54 which extends through the back wall I6 of thecasing I2 as shown in FIG. 2. The pivot 54 terminates in a readilyaccessible screw head 56 which can be rotated to adjust the position ofthe cell PC2 in the chamber 36 to in turn adjust the intensity of thelight received by the cell PC2 for a purpose to appear presently. Thehorn 40, the light source 46. and the cells PCI and PC2 are connected inacontrol circuit 60 shown in detail in FIG. 3 and the components ofwhich are housed principally in the compartment 38.

In the signal circuit 60, the detector cell PCl is connected in serieswith a resistor R1 and the cell PC2, as shown in FIG. 3. The cells PCIand PC2 are such that the resistance of each will vary generallyproportionately to the intensity of the light impinging on the cells.Direct current is supplied to the cells through conductors and I03 whichare connected to a DC power source I05 which in turn is connected to aconventional I 10 volt AC source via conductors I00 and I02. The horn40, which is a conventional I I0 volt AC horn is operative at 60 cps andis actuated via the AC lines I00 and 102 in a manner to be hereinafterdescribed.

The circuit 60 includes a four leg bridge defined by the cell PCI andresistor R1. cell PC2. resistor R2 and resistor R3. Cell PCI andresistor R1 and cell PC2 are serially connected with the combinationbeing connected in parallel with serially connected resistors R2 and R3.A resistor R13 is connected in parallel with cell PC2 for a purpose toappear presently. Bridge unbalance is sensed by a field effecttransistor amplifier O] which has its gate connected to the juncturebetween R1 and PC2 and has its drain connected between resistors R2 andR3. in this manner. when a selected threshhold of unbalance occurs inthe bridge circuit caused by a predetermined drop in the resistance ofPCI relative to the resistance of PC2. the amplifier QI will be renderedconductive. The output circuit of CI is connected to a transistoramplifier 02 such that the source of O1 is connected to the base of Q2.The amplifier O2 is a p-n-p type transistor with its emitter connectedto the DC voltage source 105 via line 103 and its base connected to thesame line via resistor R4 which acts as a biasing resistor for the baseof Q2 and load resistance for the source of 01. Thus, when 01 isrendered conductive. Q2 will also be turned on. The transistor 02provides a trigger pulse to the gate circuit of a SCR amplifier 03 so asto render amplifier O3 conductive. This is accomplished by a circuitwhich includes a resistor R8 which is connected to the gate of amplifierO3 and also to the conductor 100 via biasing resistor R5.

The resistor R4 and a parallel capacitor C1 associated with theamplifier Q2 and the resistor R5 and a parallel capacitor C2 associatedwith the SCR Q3 function as filter subcircuits to minimize thepossibility of undesirable actuation of the circuit 60 due to thepresence of transient power sources such as start-up power, lightning,and the like.

In order to provide for continuous operation of the horn 40 in responseto a sensing by PC1 of a smoke concentration above a selected threshholdlevel, a latch-up subcircuit is provided of resistors R6 and R7, diodeD1, and capacitor C3. The latch-up circuit is connected between theemitter of Q1 and the gate of O2 and operates in response to a voltagerise on capacitor C3 to hold O1 in a turned on condition.

In the operation of the unit 10, the horn 40 is operated in response toa preset differential in resistance be tween PC1 and PCZ. Thisdifferential will remain substantially constant irrespective of linevoltage changes which affect the intensity of light source 46 andtemperature conditions which affect the resistivity of PC].Consequently, PC1 is capable of sensing a threshhold condition of smokeconcentration in interaction region 46 irrespective of variableconditions such as line voltage and temperature because of thecompensating or offsetting influence of PCZ. The threshhold condition tobe set in readily adjustable to any desired level by ro tating the pivot54 to adjust the position of cell PC2 to receive more or less light fromcompartment 36.

When the intensity of light impinging on PC] relative to the intensityof light impinging on PC2 reaches the threshhold condition, sufficientcurrent can flow through Q! to trigger Q2 and in turn provide fortriggering of 03. In response to triggering of Q3 approximately halfwave drive to the horn 40 is achieved since horn 40 is connected inseries with SCR ()3. This results in the emission of a distinguishableraspy noise of high output sound level from the horn 40 and a lowtemperature rise of the horn. The resistor R1, which is of smallmagnitude compared to the resistance of PC1 when exposed to light at thethreshhold level, provides current limiting protection for O1 in casePC] shorts out. The resistor R13 in parallel with PC2 functions toinsure against an unbalance of the bridge toward turnon of ()1 after thecell PC2 has been dark for a period of time, such as after prolongedstorage of unit 10, when the circuit 60 is first connected to a powersource. The value of resistor R13 is relatively high compared to theresistance of PC2 when exposed to normal light in the unit 10.

In response to the turning on ofOl and 02 the electrical potential atthe R7, R6 node is approximately the same potential as that on acapacitor C4. After a short time, as set by the RC time constant of thecircuit of C3, the voltage on C3 rises sufficiently to hold 01 on.

thus causing latch-up. The values of R8 and R7 are low enough tomaintain the bridge unbalance in the alarm direction even when theresistance of PC] is infinite, i.e. open circuit. Once latchup isachieved, the horn 40 will continue to operate to indicate a dangeroussmoke condition until line power to conductors and 102 is removed.

When there is no line power, C4 has a zero charge. For a voltage on C4less than at the gate of Q1, diode D1 is reverse biased and the latch-upsubcircuit is isolated from the bridge and 01. Also, during a no alarm"condition, 02 is off and its collector potential is at or near line 102potential so that diode D1 is maintained in a reversed biased state. Thetime constant for the filter subcircuits is chosen so that transientsrelated to connecting power to the circuit 60, lightning, and othertypical causes will not turn on Q2.

The circuit 60 also includes a heat responsive circuit which includes aheat responsive switch 106 connected in parallel with the SCR Q3 andhence in series with the horn 40. When the switch 106 is closed by thepresence of excessive heat, the horn 40 receives full wave excitationthrough the switch 106. This produces a sound which is clearlydistinguishable from the half wave excitation previously describedindicating a threshhold smoke condition.

One of the important advantages of the circuit 60 is its ability todetect burnout of lamp 46. This is achieved by sensing a substantialvoltage rise compared with normal operation and producing short burstsof sound from horn 40 in response to this voltage rise. The lamp 46 isconnected in series with a resistor R9, a diode D5 and a resistor R10across the lines 100 and 102. The peak voltage across lamp 46 andresistor R10 is substantially less than peak AC line voltage which isthe input to the series circuit when lamp 46 is not burned out. A diodeD6 rectifies the voltage across R10 and cooperates with capacitor C4 toprovide a steady DC voltage across the amplifier and bridge circuitsthereby defining the DC power source 105. The peak voltage of lamp 46and resistor R10 is selected so that it is less than the threshhold ofthe lamp outage detection subcircuit hereinafter described.

The lamp outage detection subcircuit consists of resistors R11 and R12,capacitors C5 and C6 and voltage breakdown device which in theillustrated embodiment of the invention is a neon bulb N connected tothe gate for 03. This subcircuit constitutes a neon bulb relaxationoscillator. The bulb N is characterized by the fact that a minimumvoltage is required to cause neon breakdown and a resulting flow ofcurrent through the bulb N. In the event bulb 46 burns out, nearly fullpeak line voltage is available through conductor 107, at the bulb N.Diode D5 provides a DC current to charge C6. Each time the voltage on C6reaches the breakdown voltage of bulb N, C6 is discharged through bulb Nand a pulse is coupled to Q3 causing a half wave pulse to the horn 40.This pulse is repeated after a plurality of line frequency passes toachieve the desired intermittent emission of noise from horn 40. Thecapacitor C5 and the resistor R12 assist in controlling the phase angleof the trigger pulse to 03.

From the above description it is seen that this invention provides asmoke and heat detector 10 in which the single born 40 is employed toprovide different distinguishable alarm signals. As a result, alarmconditions such as a dangerous smoke threshhold condition and adangerous heat condition can be sensed. and a trouble condition. namely.burnout of the lamp 46 can also be sensed. As a consequence of this useof the single horn 40. an operable detector unit [0 can be produced atlow cost. in addition. by virtue of the employment of the compensatingcell PCZ in cooperation with the detector cell P(l. a smoke threshholdcondition which it is desired to detect can be set by adjustment of theposi tion of the cell PCZ. This cooperative relationship of the cellsalso compensates for variable light levels and temperature conditionswhich would otherwise adversely affect the sensitivity of the detectorcell PCl. The latch-up subcircuit assures continuous operation ofthehorn 40 when an alarm condition is sensed which is advantageous forsafety purposes because it requires the building occupant in which theunit I0 is placed to unplug the unit It) in order to shut off horn 40.

What is claimed is:

1. ln a smoke detector unit. a casing having air inlet and outletopenings. means forming a detection chamber in said casing communicatingwith said openings. a light energy source providing light in saiddetection chamber. first photosensitive cell means in said casingpositioned to receive light energy deflected from airborne particles insaid detection chamber. second photosensitive cell means in said casingfor receiving light energy from light therein. means adjustably mountingsaid second cell means on said casing so that the light energy receivedby said second cell means may be varied by adjusting its position insaid casing. said cell means being arranged relative to said lightsource such that they have a substantially uniform light intensitythereon at all light levels in said casing. said cell means beingdisposed substantially adjacent one another so that they are subjectedto the same environmental con ditions. and signal means operativelyassociated with said cell means for predetermined actuation in responseto a predetermined relationship in the light energies received by saidfirst and second cell means.

2. A smoke detector unit according to claim 1 further including acontrol circuit operatively connecting said light energy source and saidfirst and second cell means with said signal means. resistor meansconductively connected in said circuit so as to form with said cellmeans a four leg bridge in which the ratio of the electrical resistanceof said first cell means to the resistance ofsaid second cell meansdetermines the electrical con dition of said bridge. thereby providingfor actuation of said signal means when said predetermined lightrelationship occurs.

3. A smoke detector unit according to claim 1 further including acontrol circuit operatively connecting said light energy source and saidfirst and second cell means with said signal means. resistor meansconductively connected in said circuit so as to form with said cellmeans a a four leg bridge in which said first and second cell means formadjacent legs of the bridge. the other adjacent legs ofthe bridge beingformed by said resistor means. a detector branch in the circuit having aswitch amplifier therein. said branch having a first input connectedbetween said first and second cell means and a second input between saidresistor means so that a current will flow through the switch amplifierwhen the re sistance of said first cell means changes with respect tothe resistance of the second cell means in a predetermined manner. andmeans in said circuit providing for flow ofenergizing current throughsaid signal means in response to flow of current through said switchamplitier.

4. A smoke detector unit according to claim I further including meansfor actuating said signal means in response to an inoperative conditionof said light energy source. said actuating means comprising circuitmeans connecting said light energy source and said signal means andoperable in response to an increase in voltage supply to said circuitmeans due to an inoperative condition of said light energy source toactuate said signal means.

5. A smoke detector unit according to claim 4 wherein said circuit meansconstitutes an oscillator circuit providing for intermittent operationof said signal means when said oscillation circuit is operative tothereby distinguish operation of said signal means to indicate aninoperative condition of said light energy source from operation of saidsignal means to detect smoke.

6. A smoke detector unit according to claim 5 wherein said oscillatorcircuit includes a neon bulb connected in parallel with capacitor meansoperable to intermittently pass a voltage to said bulb exceeding thebreakdown voltage thereof to in turn provide for intermittent flow ofenergizing current through said bulb to said signal means.

7. In a unit for detecting a predetermined smoke concentration level inthe atmosphere. a casing. a light source in said casing. first circuitmeans for sensing a predetermined smoke condition and for providing anactuating signal in response thereto. said circuit means including afirst photosensitive cell and a second compensating photosensitive cellarranged in said casing so as to have substantially uniform distributionof light thenv on and comparable light intensity at said predeterminedsmoke level. means adjustably mounting said second cell on said casingso that the light energy received by said second cell may be varied byadjusting its position in said casing. said cells being disposedsubstantially adjacent one another so that they are sub ected to thesame environmental conditions. a switch amplifier for providing saidactuating signal when said amplifier is turned on. alarm meansresponsive to the ocunrence of said actuating signal for providing analarm signal. and a latch circuit connected to said switch amplifier andoperable to maintain said alarm means in operation after discontinuanceof said actuating signal. said latch circuit including bridge circuitmeans and means to unbalance said bridge circuit into an alarm state inresponse to turning on said switch amplifier.

8. In a smoke detecting unit. a casing having air inlet and outletopenings. means forming a detection chamber in said casing communicatingwith said openings. at light energy source providing light in saiddetection chamber, first photosensitive cell means in said casingpositioned to receive light energy deflected by airborne particles fromsaid detection chamber. a light receiving chamber positioned oppositesaid light energy source and adapted to receive light therein. secondphotosensitive cell means mounted in said light receiving chamber. saidsecond photosensitive cell means being adjustably mounted so that thelight energy recciv ed thereby may be varied by adjusting its positionwithin the light receiving chamber. said cell means being disposedsubstantially adjacent one another so that they are subjected to thesame environmental conditions. signal 7 means. a control circuitoperativel connecting said light energy source and said first and secondcell means with said signal means. and means in said circuit providingfor drive of said signal means in response to a predetermined differencein the light energies received by said first and second cell means.

9. A smoke detector unit according to claim 8 wherein said first andsecond cell means form adjacent legs of a four leg bridge. resistormeans forming the other adjacent legs of said bridge. a detector branchin said circuit having a switch amplifier therein. said branch having afirst input connected between said first and second cell means and asecond input connected between said resistor means so that the currentthrough the switch amplifier will change when the resistance of saidfirst cell means falls below the resistance of said second cell means bya predetermined amount.

[0. A smoke detector unit according to claim 8 further including a fixedresistance resistor connected in parallel with said second cell meansand having a resistance less then the resistance of said second cellmeans when said second cell means is unexposed to light and a resistancesubstantially greater than the resistance of said second cell means whensaid second cell means is exposed to light in said compartment from saidsecond light energy source.

I]. A smoke detector unit according to claim 8 further includingcapacitor means connected to said de tector branch between said switchamplifier and said cell means and connected to said conductor means sothat in response to current flow through said switch amplifier thevoltage on said capacitor means will reach a value sufficiently high tomaintain current flow through said switch amplifier so that said signalmeans will continue to operate until power to said conductor means isdiscontinued.

12. A smoke detector unit according to claim 8 wherein said lightreceiving chamber has a pair of spaced walls that are substantiallyperpendicular to the non-deflected path of light emanating from saidlight energy source. means pivotallv supporting said secondphotosensitive cell means in said light receiving chamber at a positionto one side of said path for pivotal movement toward and away from oneof said walls to thereby enable adjustment of the intensity of reflectedlight in said light receiving chamber on said second photosensitive cellmeans.

13. A smoke detector unit according to claim I wherein saidpredetermined relationship is a relationship wherein the intensity oflight on said second cell means is comparable to the light intensity onsaid first cell means.

1. In a smoke detector unit, a casing having air inlet and outletopenings, means forming a detection chamber in said casing communicatingwith said openings, a light energy source providing light in saiddetection chamber, first photosensitive cell means in said casingpositioned to receive light energy deflected from airborne particles insaid detection chamber, second photosensitive cell means in said casingfor receiving light energy from light therein, means adjustably mountingsaid second cell means on said casing so that the light energy receivedby said second cell means may be varied by adjusting its position insaid casing, said cell means being arranged relative to said lightsource such that they have a substantially uniform light intensitythereon at all light levels in said casing, said cell means beingdisposed substantially adjacent one another so that they are subjectedto the same environmental conditions, and signal means operativelyassociated with said cell means for predetermined actuation in responseto a predetermined relationship in the light energies received by saidfirst and second cell means.
 2. A smoke detector unit according to claim1 further including a control circuit operatively connecting said lightenergy source and said first and second cell means with said signalmeans, resistor means conductively connected in said circuit so as toform with said cell means a four leg bridge in which the ratio of theelectrical resistance of said first cell means to the resistance of saidsecond cell means determines the electrical condition of said bridge,thereby providing for actuation of said signal means when saidpredetermined light relationship occurs.
 3. A smoke detector unitaccording to claim 1 further including a control circuit operativelyconnecting said light energy source and said first and second cell meanswith said signal means, resistor means conductively connected in saidcircuit so as to form with said cell means a a four leg bridge in whichsaid first and second cell means form adjacent legs of the bridge, theother adjacent legs of the bridge being formed by said resistor means, adetector branch in the circuit having a switch amplifier therein, saidbranch having a first input connected between said first and second cellmeans and a second input between said resistor means so that a currentwill flow through the switch amplifier when the resistance of said firstcell means changes with respect to the resistance of the second cellmeans in a predetermined manner, and means in said circuit providing forflow of energizing current through said signal means in response to flowof current through said switch amplifier.
 4. A smoke detector unitaccording to claim 1 further including means for actuating said signalmeans in response to an inoperative condition of said light energysource, said actuating means comprising circuit means connecting saidlight energy source and said signal means and operable in response to anincrease in voltage supply to said circuit means due to an inoperativecondition of said light energy source to actuate said signal means.
 5. Asmoke detector unit according to claim 4 wherein said circuit meansconstituTes an oscillator circuit providing for intermittent operationof said signal means when said oscillation circuit is operative tothereby distinguish operation of said signal means to indicate aninoperative condition of said light energy source from operation of saidsignal means to detect smoke.
 6. A smoke detector unit according toclaim 5 wherein said oscillator circuit includes a neon bulb connectedin parallel with capacitor means operable to intermittently pass avoltage to said bulb exceeding the breakdown voltage thereof to in turnprovide for intermittent flow of energizing current through said bulb tosaid signal means.
 7. In a unit for detecting a predetermined smokeconcentration level in the atmosphere, a casing, a light source in saidcasing, first circuit means for sensing a predetermined smoke conditionand for providing an actuating signal in response thereto, said circuitmeans including a first photosensitive cell and a second compensatingphotosensitive cell arranged in said casing so as to have substantiallyuniform distribution of light thereon and comparable light intensity atsaid predetermined smoke level, means adjustably mounting said secondcell on said casing so that the light energy received by said secondcell may be varied by adjusting its position in said casing, said cellsbeing disposed substantially adjacent one another so that they aresubjected to the same environmental conditions, a switch amplifier forproviding said actuating signal when said amplifier is turned on, alarmmeans responsive to the occurrence of said actuating signal forproviding an alarm signal, and a latch circuit connected to said switchamplifier and operable to maintain said alarm means in operation afterdiscontinuance of said actuating signal, said latch circuit includingbridge circuit means and means to unbalance said bridge circuit into analarm state in response to turning on said switch amplifier.
 8. In asmoke detecting unit, a casing having air inlet and outlet openings,means forming a detection chamber in said casing communicating with saidopenings, a light energy source providing light in said detectionchamber, first photosensitive cell means in said casing positioned toreceive light energy deflected by airborne particles from said detectionchamber, a light receiving chamber positioned opposite said light energysource and adapted to receive light therein, second photosensitive cellmeans mounted in said light receiving chamber, said secondphotosensitive cell means being adjustably mounted so that the lightenergy received thereby may be varied by adjusting its position withinthe light receiving chamber, said cell means being disposedsubstantially adjacent one another so that they are subjected to thesame environmental conditions, signal means, a control circuitoperatively connecting said light energy source and said first andsecond cell means with said signal means, and means in said circuitproviding for drive of said signal means in response to a predetermineddifference in the light energies received by said first and second cellmeans.
 9. A smoke detector unit according to claim 8 wherein said firstand second cell means form adjacent legs of a four leg bridge, resistormeans forming the other adjacent legs of said bridge, a detector branchin said circuit having a switch amplifier therein, said branch having afirst input connected between said first and second cell means and asecond input connected between said resistor means so that the currentthrough the switch amplifier will change when the resistance of saidfirst cell means falls below the resistance of said second cell means bya predetermined amount.
 10. A smoke detector unit according to claim 8further including a fixed resistance resistor connected in parallel withsaid second cell means and having a resistance less then the resistanceof said second cell means when said second cell means is unexposed tolight and a resistance substantially greater than the rEsistance of saidsecond cell means when said second cell means is exposed to light insaid compartment from said second light energy source.
 11. A smokedetector unit according to claim 8 further including capacitor meansconnected to said detector branch between said switch amplifier and saidcell means and connected to said conductor means so that in response tocurrent flow through said switch amplifier the voltage on said capacitormeans will reach a value sufficiently high to maintain current flowthrough said switch amplifier so that said signal means will continue tooperate until power to said conductor means is discontinued.
 12. A smokedetector unit according to claim 8 wherein said light receiving chamberhas a pair of spaced walls that are substantially perpendicular to thenon-deflected path of light emanating from said light energy source,means pivotally supporting said second photosensitive cell means in saidlight receiving chamber at a position to one side of said path forpivotal movement toward and away from one of said walls to therebyenable adjustment of the intensity of reflected light in said lightreceiving chamber on said second photosensitive cell means.
 13. A smokedetector unit according to claim 1 wherein said predeterminedrelationship is a relationship wherein the intensity of light on saidsecond cell means is comparable to the light intensity on said firstcell means.